Fabricating non-volatile memory with boost structures

ABSTRACT

A method for fabricating a non-volatile memory having boost structures. Boost structures are provided for individual NAND strings and can be individually controlled to assist in programming, verifying and reading processes. The boost structures can be commonly boosted and individually discharged, in part, based on a target programming state or verify level. The boost structures assists in programming so that the programming and pass voltage on a word line can be reduced, thereby reducing side effects such as program disturb. During verifying, all storage elements on a word line can be verified concurrently. The boost structure can also assist during reading. In one approach, the NAND string has dual source-side select gates between which the boost structure contacts the substrate at a source/drain region, and a boost voltage is provided to the boost structure via a source-side of the NAND string.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to co-pending, commonly assigned U.S. patentapplication Ser. No. ______, filed ______, titled “OperatingNon-Volatile Memory With Boost Structures” (docket no. SAND-1126US0),and co-pending, commonly assigned U.S. patent application Ser. No.______, filed ______, titled “Non-Volatile Memory With Boost Structures”(docket no. SAND-1126US1), each of which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to non-volatile memory.

2. Description of the Related Art

Semiconductor memory has become increasingly popular for use in variouselectronic devices. For example, non-volatile semiconductor memory isused in cellular telephones, digital cameras, personal digitalassistants, mobile computing devices, non-mobile computing devices andother devices. Electrically Erasable Programmable Read Only Memory(EEPROM) and flash memory are among the most popular non-volatilesemiconductor memories. With flash memory, also a type of EEPROM, thecontents of the whole memory array, or of a portion of the memory, canbe erased in one step, in contrast to the traditional, full-featuredEEPROM.

Both the traditional EEPROM and the flash memory utilize a floating gatethat is positioned above and insulated from a channel region in asemiconductor substrate. The floating gate is positioned between thesource and drain regions. A control gate is provided over and insulatedfrom the floating gate. The threshold voltage of the transistor thusformed is controlled by the amount of charge that is retained on thefloating gate. That is, the minimum amount of voltage that must beapplied to the control gate before the transistor is turned on to permitconduction between its source and drain is controlled by the level ofcharge on the floating gate.

Some EEPROM and flash memory devices have a floating gate that is usedto store two ranges of charges and, therefore, the memory element can beprogrammed/erased between two states, e.g., an erased state and aprogrammed state. Such a flash memory device is sometimes referred to asa binary flash memory device because each memory element can store onebit of data.

A multi-state (also called multi-level) flash memory device isimplemented by identifying multiple distinct allowed/valid programmedthreshold voltage ranges. Each distinct threshold voltage rangecorresponds to a predetermined value for the set of data bits encoded inthe memory device. For example, each memory element can store two bitsof data when the element can be placed in one of four discrete chargebands corresponding to four distinct threshold voltage ranges.

Typically, a program voltage V_(PGM) applied to the control gate duringa program operation is applied as a series of pulses that increase inmagnitude over time. In one possible approach, the magnitude of thepulses is increased with each successive pulse by a predetermined stepsize, e.g., 0.2-0.4 V. V_(PGM) can be applied to the control gates offlash memory elements. In the periods between the program pulses, verifyoperations are carried out. That is, the programming level of eachelement of a group of elements being programmed in parallel is readbetween successive programming pulses to determine whether it is equalto or greater than a verify level to which the element is beingprogrammed. For arrays of multi-state flash memory elements, averification step may be performed for each state of an element todetermine whether the element has reached its data-associated verifylevel. For example, a multi-state memory element capable of storing datain four states may need to perform verify operations for three comparepoints.

Moreover, when programming an EEPROM or flash memory device, such as aNAND flash memory device in a NAND string, typically V_(PGM) is appliedto the control gate and the bit line is grounded, causing electrons fromthe channel of a cell or memory element, e.g., storage element, to beinjected into the floating gate. When electrons accumulate in thefloating gate, the floating gate becomes negatively charged and thethreshold voltage of the memory element is raised so that the memoryelement is considered to be in a programmed state. More informationabout such programming can be found in U.S. Pat. No. 6,859,397, titled“Source Side Self Boosting Technique For Non-Volatile Memory,” and inU.S. Patent Application Pub. 2005/0024939, titled “Detecting OverProgrammed Memory,” published Feb. 3, 2005; both of which areincorporated herein by reference in their entirety.

However, during programming, various problems such as program disturbcan occur due to the need to apply relatively high voltages to the wordlines. Accordingly, there is a continuing need to further optimizeprogramming operations as well as verify and read operations.

SUMMARY OF THE INVENTION

The present invention addresses the above and other issues by providinga method for fabricating non-volatile memory with boost structures.

In one embodiment, a method for fabricating non-volatile storageincludes forming a set of storage elements, at least in part, on asubstrate, forming inner and outer select gates at a first end of theset of storage elements and forming a boost structure extending alongthe set of storage elements and in communication with the substrate at alocation between the inner and outer select gates.

When operating the non-volatile storage, after a boost voltage isapplied to the boost structure, the boost structure can be furtherboosted by applying an elevated voltage to word lines associated withthe NAND string. Subsequently, the boost structure can be discharged, atleast in part, to a level which is based on a programming state to whicha storage element is to be programmed. The level to which the booststructure is discharged can be controlled by controlling a voltageapplied to a drain side of the NAND string, e.g., via a bit line. Afterthe discharging, programming can occur by applying a programming voltageto a storage element in the NAND string, e.g., via a selected word line.Advantageously, the boost structure assists in the programming processso that a lower programming voltage can be applied on the selected wordline.

In a verify process, which can occur between programming pulses, theboost structure can be boosted again and discharged independently foreach NAND string to a level which is based on a verify level. A voltageis then applied to a storage element in each NAND string to characterizea programming state of the storage element. With this approach, multiplestorage elements along a word line in different NAND strings can beconcurrently characterized based on different verify levels while acommon word line voltage is used.

A read process can involve a number of read cycles, one for each readlevel. In each read cycle, the boost structures are commonly boosted anddischarged to a common level which is based on a read level whichdiffers in each read cycle.

In another embodiment, a method for fabricating non-volatile storageincludes forming a number of NAND strings on a substrate, forming innerand outer select gates at a first end of each NAND string and forming anumber of boost structures. Each boost structure extends along arespective NAND string and communicates with the substrate at arespective location between the first and second select gates of therespective NAND string.

In another embodiment, a method for fabricating non-volatile storageincludes forming a number of NAND strings on a substrate, forming anumber of boost structures, each boost structure extending along arespective NAND string and forming control lines for concurrentlyapplying a common boost voltage to each boost structure, and forindependently discharging each boost structure, at least in part.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view of two adjacent NAND strings with boost structures.

FIG. 2 is an equivalent circuit diagram of the NAND strings of FIG. 1.

FIG. 3 is a top view of an alternative embodiment of two adjacent NANDstrings with boost structures.

FIG. 4 is an equivalent circuit diagram of the NAND strings of FIG. 3.

FIG. 5 is a circuit diagram depicting three NAND strings with dualsource-side select gates and boost structures.

FIGS. 6-10 depict fabrication of non-volatile storage with booststructures.

FIG. 6 depicts non-volatile storage.

FIG. 7 depicts details of a portion of FIG. 6.

FIG. 8 depicts the non-volatile storage of FIG. 6 after depositing aninsulating layer.

FIG. 9 a depicts the non-volatile storage of FIG. 8 after depositing aphoto resist layer.

FIG. 9 b depicts the non-volatile storage of FIG. 9 a after selectivelyexposing and removing photo resist, and etching portions of theinsulating layer.

FIG. 10 depicts the non-volatile storage of FIG. 9 b after depositing aconductive layer which provides a boost structure.

FIG. 11 is a top view of another embodiment of two adjacent NAND stringswith boost structures.

FIG. 12 is an equivalent circuit diagram of the NAND strings of FIG. 11.

FIG. 13 is a circuit diagram depicting three NAND strings with singlesource-side select gates and boost structures.

FIG. 14 depicts a process for fabricating non-volatile storage havingboost structures.

FIG. 15 is a block diagram of an array of NAND flash storage elements.

FIG. 16 is a block diagram of a non-volatile memory system using singlerow/column decoders and read/write circuits.

FIG. 17 is a block diagram of a non-volatile memory system using dualrow/column decoders and read/write circuits.

FIG. 18 is a block diagram depicting one embodiment of a sense block.

FIG. 19 illustrates an example of an organization of a memory array intoblocks for an all bit line memory architecture or for an odd-even memoryarchitecture.

FIG. 20 depicts an example set of threshold voltage distributions.

FIG. 21 depicts another example set of threshold voltage distributions.

FIGS. 22 a-c show various threshold voltage distributions and describe aprocess for programming non-volatile memory.

FIG. 23 depicts a timing diagram describing a process for programmingnon-volatile memory.

FIG. 24 depicts a timing diagram describing a process for verifying aprogramming state of non-volatile memory.

FIG. 25 is a graph depicting a bit line voltage versus time relationshipfor different programming states.

FIG. 26 depicts an example waveform applied to the control gates ofnon-volatile storage elements during programming.

FIG. 27 depicts a timing diagram describing a process for reading aprogramming state of non-volatile memory.

FIG. 28 depicts a timing diagram for unselected bit lines duringprogramming, verifying or reading of non-volatile memory.

FIG. 29 is a flow chart describing one embodiment of a process forprogramming non-volatile memory.

FIG. 30 is a flow chart describing one embodiment of a process forreading a non-volatile memory.

DETAILED DESCRIPTION

One example of a non-volatile memory system suitable for use with thepresent invention uses a NAND flash memory structure, in which multipletransistors are arranged in series between select gates in a NANDstring. FIG. 1 is a top view showing two NAND strings arranged one afteranother. In practice, a number of such NAND strings can be arranged oneafter another in a two-dimensional array across a semiconductor deviceand, optionally, in three dimensions. The NAND strings depicted in FIGS.1 and 2 each include four transistors in series and sandwiched betweenselect gates. In one embodiment, each NAND string includes twosource-side select gates and one drain-side select gate. For exampleNAND string #1 includes transistors 100, 102, 104 and 106 sandwichedbetween a drain-side select gate (not shown) and source-side selectgates 108 and 110. Select gates 108 and 110 can be connected to, orprovided as part of, control lines SGS2 and SGS1, respectively, of theassociated NAND string. NAND string #2 includes transistors 134, 136,138 and 140 sandwiched between source-side select gates 130 and 132 anda drain-side select gate 142. Select gates 130 and 132 can be connectedto, or provided as part of, control lines SGS1 and SGS2, respectively,of the associated NAND string. Note that the depiction of one end regionof NAND string #1 has been cut off on the drain side. Select gates 108and 132 may be considered to be inner select gates and select gates 110and 130 may be considered to be outer select gates based on theirposition relative to the respective NAND string.

In NAND string #1, for instance, a drain-side select gate (not shown)connects the NAND string to a bit line contact (not shown) on one endand the select gate 110 connects the NAND string to a source linecontact 120 on the other end. Similarly, in NAND string #2, select gate142 connects the NAND string to a bit line contact 150 on one end andthe select gate 130 connects the NAND string to the source line contact120 on the other end. The select gates are controlled by applyingappropriate voltages via their control lines. In this approach, the NANDstrings are arranged alternately source-side to source-side anddrain-side to drain-side in a bit line direction. However, otherapproaches can also be used.

Further, in NAND string #1, each of the transistors 100, 102, 104 and106 has a control gate and a floating gate. Specifically, referring toFIG. 2, transistor 100 has control gate 100CG and floating gate 100FG.Transistor 102 includes control gate 102CG and floating gate 102FG.Transistor 104 includes control gate 104CG and floating gate 104FG.Transistor 106 includes a control gate 106CG and floating gate 106FG.Control gates 100CG, 102CG, 104CG and 106CG can be provided as portionsof word lines WL3, WL2, WL1 and WL0, respectively. In one possibledesign, transistors 100, 102, 104 and 106 are each memory cells ornon-volatile storage elements. In other designs, the memory elements mayinclude multiple transistors or may be different than those depicted inFIGS. 1 and 2. Similarly, in NAND String #2, each of the transistors134, 136, 138 and 140 has a control gate and a floating gate. Transistor134 has control gate 134CG and floating gate 134FG. Transistor 136includes control gate 136CG and floating gate 136FG. Transistor 138includes control gate 138CG and floating gate 138FG. Transistor 140includes a control gate 140CG and floating gate 140FG. Control gates134CG, 136CG, 138CG and 140CG can be provided as portions of associatedword lines WL0, WL1, WL2 and WL3, respectively. These are different wordlines than those associated with NAND string #1.

In one possible implementation, F denotes the width of the word line,control gate and the floating gate of each memory element, as well asthe spacing between memory elements. The source and drain select gatesmay also have a width of F. The spacing between the source-side selectgates can be F or multiples of F, for instance. The use of dualsource-side select gates is useful in preventing current leakage at thesource side of the NAND through the select gates as well as incontrolling a boost structure.

In particular, a boost structure such as a conductive plate or line canbe provided for each NAND string, in one approach. NAND string #1includes a boost structure 115 which extends along the NAND string andits storage elements and contacts the NAND string at a source/drainregion between the source-side select gates 108 and 110 via a contact tosubstrate 118, in one approach. The boost structure 115 is thus incommunication with the substrate. As described further below in detail,the boost structure can assist in programming, verify and/or readoperations to optimize these processes for individual NAND strings.Similarly, NAND string #2 includes a boost structure 145 which extendsalong the NAND string and its storage elements and contacts the NANDstring at a source/drain region between the source-side select gates 130and 132 via a contact to substrate 148, in one approach. Other booststructure configurations can also be provided, such as a boost structurewhich is shared by one or more adjacent NAND strings in a word linedirection and/or bit line direction. For example, the boost structuremay extend over adjacent NAND strings which are programmed at differenttimes using odd-even programming. Such a boost structure may contactsource/drain regions in the adjacent NAND strings. In another approach,multiple boost structures are associated with one NAND string. Further,other boost structures need not contact a source/drain region of thesubstrate but can be controlled via other means. Also, other booststructures can contact the substrate at locations other than between twoor more source-side select gates. A boost structure can similarly beused with a NAND string having a single source-side or drain-side selectgate.

In another variation, a boost structure can contact a NAND string on adrain side, e.g., near or adjacent to the drain-side select gate orbetween two or more drain-side select gates which are provided byanalogy to the two source-side select gates of FIGS. 1 and 2. The booststructure can have various cross-sectional shapes. The boost structurecan conform to the shape of the NAND string or can extend in a straightline. The boost structure can have various cross sectional shapes alongits length, including rectangular like a thin strip, circular like awire, and so forth. Further, the cross section can very along thelength. For example, the cross section can be thicker in some locationsthan others. Also, the boost structure can extend along a portion of aNAND string, e.g., along only a portion of the storage elements, oralong all of the storage elements of a NAND string. Other variations arealso possible.

FIG. 3 is a top view of an alternative embodiment of two adjacent NANDstrings. FIG. 4 is an equivalent circuit diagram of the NAND strings ofFIG. 3. As mentioned, the spacing between the source-side select gatescan be F or integer multiples of F, in one approach. In the presentexample, the spacing is 3F between the source-side select gates 108 and110 in NAND string #1, and between source-side select gates 130 and 132in NAND string #2. This approach can ease tolerances for locating theboost structures' contacts to the substrate 118 or 148, relative to theembodiment of FIGS., 1 and 2 which employs a spacing of F. Inparticular, larger, non-self-aligned contacts can be used for couplingthe boost structure to the active area of the substrate, e.g., thesource/drain regions.

FIG. 5 is a circuit diagram depicting three NAND strings. The circuitdiagram corresponds to the embodiment of FIGS. 1-4. A typicalarchitecture for a flash memory system using a NAND structure willinclude several NAND strings. For example, three NAND strings 520, 540and 560 are shown in a memory array having many more NAND strings. Inthis example, each of the NAND strings includes two source-side selectgates, four storage elements and one drain-side select gate. While fourstorage elements are illustrated for simplicity, modem NAND strings canhave up to thirty-two or sixty-four storage elements, for instance.

For example, NAND string 520 includes drain-side select gate 522,storage elements 523-526, source-side select gates 527 and 528, andboost structure 530 with a contact to substrate 531. NAND string 540includes drain-side select gate 542, storage elements 543-546,source-side select gates 547 and 548, and boost structure 550 with acontact to substrate 551. NAND string 560 includes drain-side selectgate 562, storage elements 563-566, source-side select gates 567 and568, and boost structure 570 with a contact to substrate 571. Each NANDstring is connected to the source line by a select gate. Selection linesor control lines SGS1 and SGS2 are used to control the source-sideselect gates. The various NAND strings 520, 540 and 560 are connected torespective bit lines 521, 541 and 561 by select transistors in theselect gates 522, 542, 562, etc. These select transistors are controlledby a drain select line or control line SGD. In other embodiments, theselect lines do not necessarily need to be in common among the NANDstrings; that is, different select lines can be provided for differentNAND strings. Word line WL3 is connected to the control gates forstorage elements 523, 543 and 563. Word line WL2 is connected to thecontrol gates for storage elements 524, 544 and 564. Word line WL1 isconnected to the control gates for storage elements 525, 545 and 565.Word line WL0 is connected to the control gates for storage elements526, 546 and 566. As can be seen, each bit line and the respective NANDstring comprise the columns of the array or set of storage elements. Theword lines (WL3, WL2, WL1 and WL0) comprise the rows of the array orset. Each word line connects the control gates of each storage elementin the row. Or, the control gates may be provided by the word linesthemselves. For example, word line WL2 provides the control gates forstorage elements 524, 544 and 564. In practice, there can be thousandsof storage elements on a word line.

In one embodiment, data is programmed to storage elements along a commonword line. Thus, prior to applying the program pulses, one of the wordlines is selected for programming. This word line is referred to as theselected word line. The remaining word lines of a block are referred toas the unselected word lines. The selected word line may have one or twoneighboring word lines. If the selected word line has two neighboringword lines, then the neighboring word line on the drain side is referredto as the drain-side neighboring word line and the neighboring word lineon the source side is referred to as the source-side neighboring wordline. For example, if WL2 is the selected word line, then WL1 is thesource-side neighboring word line and WL3 is the drain-side neighboringword line.

Each block of storage elements includes a set of bit lines formingcolumns and a set of word lines forming rows. In one embodiment, the bitlines are divided into odd bit lines and even bit lines. As discussedalso in connection with FIG. 19, storage elements along a common wordline and connected to the odd bit lines are programmed at one time,while storage elements along a common word line and connected to evenbit lines are programmed at another time (“odd/even programming”). Inanother embodiment, storage elements are programmed along a word linefor all bit lines in the block (“all bit line programming”). In otherembodiments, the bit lines or block can be broken up into othergroupings (e.g., left and right, more than two groupings, etc.)

Further, each storage element can store data. For example, when storingone bit of digital data, the range of possible threshold voltages(V_(TH)) of the storage element is divided into two ranges which areassigned logical data “1” and “0.” In one example of a NAND type flashmemory, the V_(TH) is negative after the storage element is erased, anddefined as logic “1.” The V_(TH) after a program operation is positiveand defined as logic “0.” When the V_(TH) is negative and a read isattempted, the storage element will turn on to indicate logic “1” isbeing stored. When the V_(TH) is positive and a read operation isattempted, the storage element will not turn on, which indicates thatlogic “0” is stored. A storage element can also store multiple levels ofinformation, for example, multiple bits of digital data. In this case,the range of V_(TH) value is divided into the number of levels of data.For example, if four levels of information are stored, there will befour V_(TH) ranges assigned to the data values “11”, “10”, “01”, and“00.” In one example of a NAND type memory, the V_(TH) after an eraseoperation is negative and defined as “11”. Positive V_(TH) values areused for the states of “10”, “01”, and “00.” The specific relationshipbetween the data programmed into the storage element and the thresholdvoltage ranges of the element depends upon the data encoding schemeadopted for the storage elements. For example, U.S. Pat. No. 6,222,762and U.S. Patent Application Pub. 2004/0255090, both of which areincorporated herein by reference in their entirety, describe variousdata encoding schemes for multi-state flash storage elements.

Relevant examples of NAND type flash memories and their operation areprovided in U.S. Pat. Nos. 5,386,422, 5,522,580, 5,570,315, 5,774,397,6,046,935, 6,456,528 and 6,522,580, each of which is incorporated hereinby reference.

When programming a flash storage element, a program voltage is appliedto the control gate of the storage element and the bit line associatedwith the storage element is grounded. Electrons from the channel areinjected into the floating gate. When electrons accumulate in thefloating gate, the floating gate becomes negatively charged and theV_(TH) of the storage element is raised. To apply the program voltage tothe control gate of the storage element being programmed, that programvoltage is applied on the appropriate word line. As discussed above, onestorage element in each of the NAND strings share the same word line.For example, when programming storage element 524 of FIG. 5, the programvoltage will also be applied to the control gates of storage elements544 and 564.

However, shifts in the charged stored in a storage element can occurwhen programming and reading a given storage element and other storageelements which have some degree of coupling with the given storageelement, such as those sharing the same word line or bit line.Specifically, shifts in the stored charge levels occur because of fieldcoupling between storage elements. The problem is exacerbated as thespaces between storage elements are being decreased due to improvementsin integrated circuit manufacturing techniques. The problem occurs mostmarkedly between two groups of adjacent storage elements that have beenprogrammed at different times. One group of storage elements isprogrammed to add a level of charge that corresponds to one set of data.After a second group of storage elements is programmed with a second setof data, the charge levels read from the first group of storage elementsoften appear to be different than what was programmed due to capacitivecoupling of the charges of the second group of storage elements to thefirst group of storage elements. Thus, the effects of coupling depend onthe order in which the storage elements are programmed and, therefore,the order in which the word lines are traversed during programming. ANAND string is typically, but not always, programmed from the sourceside to the drain side, starting at the source-side word line andproceeding, one word line at a time, to the drain-side word line.

Capacitive coupling effects on a given storage element can be caused byother storage elements in the same word line and in the same NANDstring, for instance. For example, storage element 544 may be part of afirst group of storage elements, which includes other alternatingstorage elements along word line WL2, which store a page of data.Storage elements 524 and 564 may be part of a second group of storageelements which store another page of data. When the second group ofstorage elements is programmed after storage element 544, there will bea capacitive coupling to storage element 544. The coupling is strongestfrom the direct neighboring storage elements on the word line, which arestorage elements 524 and 564.

Similarly, storage element 544 can be affected by programming of storageelements which are on the same NAND string 540 if they are programmedafter storage element 544. For storage element 544, the coupling isstrongest from the direct neighboring storage elements on the NANDstring, which are storage elements 543 and/or 545. For example, ifstorage elements in the NAND string 540 are programmed in the order:546, 545, 544, 543, storage element 544 can be affected by coupling fromstorage element 543. Generally, storage elements which are arrangeddiagonally with respect to storage element 544, namely storage elements523, 563, 525 and 565, can provide about 20% of the coupling for storageelement 544, whereas the direct neighboring storage elements 524 and564, and 543 and 545 on the same word line or NAND string provide about80% of the coupling. The coupling may be enough to shift the V_(TH) of astorage element by about 0.5 V in some cases, which is sufficient tocause a read error and to widen the V_(TH) distribution of a group ofstorage elements.

FIGS. 6-10 depict fabrication of non-volatile storage with booststructures. FIG. 6 depicts non-volatile storage. A cross-sectional viewalong a number of NAND strings is shown. A corresponding structure canbe repeated further to the right and left along a bit line direction aswell as in a word line direction which extends out of the page. Thedepiction is simplified and does not necessarily provide all details,nor is the depiction necessarily to scale. In the configurationprovided, the NAND strings are arranged alternatingly source-side tosource-side and drain-side to drain-side. However, other configurationsare possible. The NAND strings can be formed on a substrate 600, atleast in part. In one approach, the NAND strings can be formed on ap-well region which is above an n-well region of a p-type substrate.

For example, a partial NAND string 605 includes source-side select gates620 and 622 which are connected to, or provided as part of, controllines SGS2 and SGS1, respectively, for the associated NAND string. Afull NAND string 610 includes source-side select gates 626 and 628 whichare connected to, or provided as part of, control lines SGS2 and SGS1,respectively, for the associated NAND string. Also provided are storageelements 630, 632, 634 and 636 and drain-side select gate 638. A partialNAND string 615 includes a drain-side select gate 642 which is connectedto, or provided as part of, a control lines SGD for the associated NANDstring. Each of the NAND strings 605, 610 and 615 may be located indifferent blocks of a memory array which are programmed or read atdifferent times, in one approach. Furthermore, source/drain diffusionregions are provided between the storage elements and select gates,including source/drain diffusion regions 650, 652, 654, 656, 658, 660,662, 664 and 666.

A source line 624 is a control line for providing a voltage to thesource-side select gates 622 and 626, while a bit line 640 is a controlline for providing a voltage to the drain-side select gates 638 and 642.

FIG. 7 depicts details of a portion of FIG. 6. Each storage element caninclude a control gate 635, floating gate 639, inter-poly dielectriclayer 637 between the control gate and floating gate, and insulatinglayer 641 between the floating gate 639 and the substrate 600.

FIG. 8 depicts the non-volatile storage of FIG. 6 after depositing aninsulating layer. An insulating layer 800 can be deposited using varioustechniques. In one approach, the layer extends across each of the NANDstrings although in practice it can be sufficient for the insulatinglayer to extend across the storage elements. Generally, the insulatinglayer 800 can conform to the shape of the existing structures includingthe storage elements and select gates. In one approach, the insulatinglayer 800 can include a triple layer dielectric formed of silicon oxide,silicon nitride and silicon oxide (“ONO”).

FIG. 9 a depicts the non-volatile storage of FIG. 8 after depositing aphoto resist layer 905. FIG. 9 b depicts the non-volatile storage ofFIG. 9 a after selectively exposing and removing portions of the photoresist layer 905, and etching portions of the insulating layer. In thisapproach, a mask 900 for selectively exposing the photo resist is usedwhich has openings between the source-side select gates of the NANDstrings. Once the photo resist layer 905 is selectively exposed, themask 900 is removed, and, the exposed portions of the photo resist areremoved using known techniques. As a result, portions of the insulatinglayer beneath the removed portions of the photo resist are exposed.Further, an etch is performed to remove these exposed portions of theinsulating layer 800 so that corresponding portions 910 and 920 of thesource/drain regions 650 and 654, respectively, are exposed. Asmentioned, e.g., in connection with FIG. 3, the spacing between thesource-side select gates can be a multiple of F to ease tolerances forlocating the boost structures' contacts to the substrate. The exampleprovided uses a source-side select gate spacing of F. Following theetch, the remainder of the photo resist layer 905 is removed, and aconductive layer is deposited.

FIG. 10 depicts the non-volatile storage of FIG. 9 b after depositing aconductive layer which provides a boost structure. A mask 1000 can beused to deposit a conductive material such as polysilicon or a metalsuch as Tungsten, Tantalum Nitride or Titanium Nitride over the NANDstrings. The conductive material can be deposited using atomic layerdeposition (ALD), chemical vapor deposition (CVD), physical vapordeposition (PVD), sputtering, etc. The conductive material provides aboost structure (partial) 1010 which contacts the substrate atsource/drain region portion 910, a boost structure 1020 which contactsthe substrate at source/drain region portion 920 and a boost structure(partial) 1030. Generally, the boost structures can conform to the shapeof the insulating layer 800. The insulating layer 800 serves to insulatethe control gates and floating gates of the storage elements from theboost structure. Furthermore, one or more additional layers, such as asecond insulating layer, can be provided on top of the boost structures.

One benefit of the boost structure is that it can provide shieldingbetween storage elements to reduce coupling effects since the booststructure can wrap around the control gates and floating gates,extending close to the substrate between the floating gates of adjacentstorage elements (FIG. 10). This design also improves coupling from theboost structure to the floating gates of the storage elements.

FIG. 11 is a top view of another embodiment of two adjacent NAND stringswith boost structures. FIG. 12 is an equivalent circuit diagram of theNAND strings of FIG. 11. In this embodiment, the NAND strings include asingle source-side select gate, e.g., select gate 108 in NAND string #1and select gate 132 in NAND string #2. A boost structure 1115 in NANDstring #1 has a contact to substrate 1118 between the select gate 108and the adjacent storage element 106, in one possible approach.Similarly, a boost structure 1145 in NAND string #2 has a contact tosubstrate 1148 between the select gate 132 and the adjacent storageelement 134, in one possible approach.

FIG. 13 is a circuit diagram depicting three NAND strings with singlesource-side select gates and boost structures. The circuit diagramcorresponds to the embodiment of FIGS. 11 and 12, and differs from thecircuit diagram of FIG. 5 in that the NAND strings include onesource-side select gate rather than two. Here, NAND string 1320 includesdrain-side select gate 1322, storage elements 1323-1326, source-sideselect gate 1327, and boost structure 1330 with a contact to substrate1331. NAND string 1340 includes drain-side select gate 1342, storageelements 1343-1346, source-side select gate 1347, and boost structure1350 with a contact to substrate 1351. NAND string 1360 includesdrain-side select gate 1362, storage elements 1363-1366, source-sideselect gate 1367, and boost structure 1370 with a contact to substrate1371. Each NAND string is connected to the source line by a select gate.Selection line or control line SGS is used to control the source-sideselect gates. The various NAND strings 1320, 1340 and 1360 are connectedto respective bit lines 1321, 1341 and 1361, by select transistors inthe select-side gates 1322, 1342, 1362, etc. These select transistorsare controlled by a drain select line or control line SGD. The NANDstrings can be controlled in a manner which is analogous to theembodiment in which the NAND strings include dual select gates at oneend. For instance, in one approach, charging of the boost structures canoccur via the bit lines.

FIG. 14 depicts a process for fabricating non-volatile storage havingboost structures. Note that in this and other flowcharts providedherein, the steps performed are not necessarily performed in the ordershown or as discrete steps. Moreover, the process provides a high-leveloverview. Step 1400 includes forming storage elements and select gateson a substrate (FIG. 6). In one embodiment, NAND strings are formed.Step 1410 includes forming source-drain regions between the storageelements and select gates. Step 1420 includes forming control lines. Forexample, these may include control lines which control the source- anddrain-side select gates and control lines which are coupled to thesource contacts and bit lines. Step 1430 includes forming a conforminginsulating layer over the storage elements and at least a portion of theselect gates (FIG. 8). Step 1440 includes forming a photo resist layer(FIG. 9 a). Step 1450 includes selectively exposing the photo resistlayer, e.g., via mask 900, and removing the exposed portions of thephoto resist to expose portions of the insulating layer (FIG. 9 b). Step1460 includes etching the exposed portions of the insulating layer toexpose portions of the source/drain regions (FIG. 9 b). Step 1470includes removing the remaining photo resist. Step 1480 includes forminga conforming conductive layer, portions of which contact the exposedportions of the source-drain regions (FIG. 10). An additional insulatinglayer can be provided over the conductive layer as well.

FIG. 15 illustrates an example of an array 1500 of NAND storageelements, such as those shown in FIGS. 1-4, 11 and 12. Along eachcolumn, a bit line 1506 is coupled to the drain terminal 1526 of thedrain select gate for the NAND string 1550. Along each row of NANDstrings, a source line 1504 may connect all the source terminals 1528 ofthe source-side select gates of the NAND strings. An example of a NANDarchitecture array and its operation as part of a memory system is foundin U.S. Pat. Nos. 5,570,315; 5,774,397; and 6,046,935.

The array of storage elements is divided into a large number of blocksof storage elements. As is common for flash EEPROM systems, the block isthe unit of erase. That is, each block contains the minimum number ofstorage elements that are erased together. Each block is typicallydivided into a number of pages. A page is a unit of programming. In oneembodiment, the individual pages may be divided into segments and thesegments may contain the fewest number of storage elements that arewritten at one time as a basic programming operation. One or more pagesof data are typically stored in one row of storage elements. A page canstore one or more sectors. A sector includes user data and overheaddata. Overhead data typically includes an Error Correction Code (ECC)that has been calculated from the user data of the sector. A portion ofthe controller (described below) calculates the ECC when data is beingprogrammed into the array, and also checks it when data is being readfrom the array. Alternatively, the ECCs and/or other overhead data arestored in different pages, or even different blocks, than the user datato which they pertain.

A sector of user data is typically 512 bytes, corresponding to the sizeof a sector in magnetic disk drives. Overhead data is typically anadditional 16-20 bytes. A large number of pages form a block, anywherefrom 8 pages, for example, up to 32, 64, 128 or more pages. In someembodiments, a row of NAND strings comprises a block.

Memory storage elements are erased in one embodiment by raising thep-well to an erase voltage (e.g., 20 V) for a sufficient period of timeand grounding the word lines of a selected block while the source andbit lines are floating. Due to capacitive coupling, the unselected wordlines, bit lines, select lines, and c-source are also raised to asignificant fraction of the erase voltage. A strong electric field isthus applied to the tunnel oxide layers of selected storage elements andthe data of the selected storage elements are erased as electrons of thefloating gates are emitted to the substrate side, typically byFowler-Nordheim tunneling mechanism. As electrons are transferred fromthe floating gate to the p-well region, the threshold voltage of aselected storage element is lowered. Erasing can be performed on theentire memory array, separate blocks, or another unit of storageelements.

FIG. 16 is a block diagram of a non-volatile memory system using singlerow/column decoders and read/write circuits. The memory device 1696 hasread/write circuits for reading and programming a page of storageelements in parallel, according to one embodiment of the presentinvention. Memory device 1696 may include one or more memory die 1698.Memory die 1698 includes a two-dimensional array of storage elements1500, control circuitry 1610, and read/write circuits 1665. In someembodiments, the array of storage elements can be three dimensional. Thememory array 1500 is addressable by word lines via a row decoder 1630and by bit lines via a column decoder 1660. The read/write circuits 1665include multiple sense blocks 1600 and allow a page of storage elementsto be read or programmed in parallel. Typically a controller 1650 isincluded in the same memory device 1696 (e.g., a removable storage card)as the one or more memory die 1698. Commands and Data are transferredbetween the host and controller 1650 via lines 1620 and between thecontroller and the one or more memory die 1698 via lines 1618.

The control circuitry 1610 cooperates with the read/write circuits 1665to perform memory operations on the memory array 1500. The controlcircuitry 1610 includes a state machine 1612, an on-chip address decoder1614 and a power control module 1619. The state machine 1612 provideschip-level control of memory operations. The on-chip address decoder1614 provides an address interface between that used by the host or amemory controller to the hardware address used by the decoders 1630 and1660. The power control module 1616 controls the power and voltagessupplied to the word lines and bit lines during memory operations.

In some implementations, some of the components can be combined. Invarious designs, one or more of the components of (alone or incombination), other than storage element array 1500, can be thought ofas a managing circuit. For example, one or more managing circuits mayinclude any one of or a combination of control circuitry 1610, statemachine 1612, decoders 1614, 1630 and 1660, power control 1616, senseblocks 1600, read/write circuits 1665, controller 1650, etc.

FIG. 17 is a block diagram of a non-volatile memory system using dualrow/column decoders and read/write circuits. Another arrangement of thememory device 1696 shown in FIG. 16 is provided. Here, access to thememory array 1500 by the various peripheral circuits is implemented in asymmetric fashion, on opposite sides of the array, so that the densitiesof access lines and circuitry on each side are reduced by half. Thus,the row decoder is split into row decoders 1630A and 1630B and thecolumn decoder into column decoders 1660A and 1660B. Similarly, theread/write circuits are split into read/write circuits 1665A connectingto bit lines from the bottom and read/write circuits 1665B connecting tobit lines from the top of the array 1500. In this way, the density ofthe read/write modules is essentially reduced by one half. The device ofFIG. 17 can also include a controller, as described above for the deviceof FIG. 16.

FIG. 18 is a block diagram of an individual sense block 1600 partitionedinto a core portion, referred to as a sense module 1680, and a commonportion 1690. In one embodiment, there will be a separate sense module1680 for each bit line and one common portion 1690 for a set of multiplesense modules 1680. In one example, a sense block will include onecommon portion 1690 and eight sense modules 1680. Each of the sensemodules in a group will communicate with the associated common portionvia a data bus 1672. For further details refer to U.S. PatentApplication Pub. No. 2006/0140007, title “Non-Volatile Memory & Methodwith Shared Processing for an Aggregate of Sense Amplifiers” publishedJun. 29, 2006, incorporated herein by reference in its entirety.

Sense module 1680 comprises sense circuitry 1670 that determines whethera conduction current in a connected bit line is above or below apredetermined threshold level. Sense module 1680 also includes a bitline latch 1682 that is used to set a voltage condition on the connectedbit line. For example, a predetermined state latched in bit line latch1682 will result in the connected bit line being pulled to a statedesignating program inhibit (e.g., V_(DD)).

Common portion 1690 comprises a processor 1692, a set of data latches1694 and an I/O Interface 1696 coupled between the set of data latches1694 and data bus 1620. Processor 1692 performs computations. Forexample, one of its functions is to determine the data stored in thesensed storage element and store the determined data in the set of datalatches. The set of data latches 1694 is used to store data bitsdetermined by processor 1692 during a read operation. It is also used tostore data bits imported from the data bus 1620 during a programoperation. The imported data bits represent write data meant to beprogrammed into the memory. I/O interface 1696 provides an interfacebetween data latches 1694 and the data bus 1620.

During read or sensing, the operation of the system is under the controlof state machine 1612 that controls the supply of different control gatevoltages to the addressed storage element. As it steps through thevarious predefined control gate voltages corresponding to the variousmemory states supported by the memory, the sense module 1680 may trip atone of these voltages and an output will be provided from sense module1680 to processor 1692 via bus 1672. At that point, processor 1692determines the resultant memory state by consideration of the trippingevent(s) of the sense module and the information about the appliedcontrol gate voltage from the state machine via input lines 1693. Itthen computes a binary encoding for the memory state and stores theresultant data bits into data latches 1694. In another embodiment of thecore portion, bit line latch 1682 serves double duty, both as a latchfor latching the output of the sense module 1680 and also as a bit linelatch as described above.

It is anticipated that some implementations will include multipleprocessors 1692. In one embodiment, each processor 1692 will include anoutput line (not depicted in FIG. 15) such that each of the output linesis wired-OR'd together. In some embodiments, the output lines areinverted prior to being connected to the wired-OR line. Thisconfiguration enables a quick determination during the programverification process of when the programming process has completedbecause the state machine receiving the wired-OR can determine when allbits being programmed have reached the desired level. For example, wheneach bit has reached its desired level, a logic zero for that bit willbe sent to the wired-OR line (or a data one is inverted). When all bitsoutput a data zero (or a data one inverted), then the state machineknows to terminate the programming process. Because each processorcommunicates with eight sense modules, the state machine needs to readthe wired-OR line eight times, or logic is added to processor 1692 toaccumulate the results of the associated bit lines such that the statemachine need only read the wired-OR line one time. Similarly, bychoosing the logic levels correctly, the global state machine can detectwhen the first bit changes its state and change the algorithmsaccordingly.

During program or verify, the data to be programmed is stored in the setof data latches 1694 from the data bus 1620. The program operation,under the control of the state machine, comprises a series ofprogramming voltage pulses applied to the control gates of the addressedstorage elements. Each programming pulse is followed by a read back(verify) to determine if the storage element has been programmed to thedesired memory state. Processor 1692 monitors the read back memory staterelative to the desired memory state. When the two are in agreement, theprocessor 1692 sets the bit line latch 1682 so as to cause the bit lineto be pulled to a state designating program inhibit. This inhibits thestorage element coupled to the bit line from further programming even ifprogramming pulses appear on its control gate. In other embodiments theprocessor initially loads the bit line latch 1682 and the sensecircuitry sets it to an inhibit value during the verify process.

Data latch stack 1694 contains a stack of data latches corresponding tothe sense module. In one embodiment, there are three data latches persense module 1680. In some implementations (but not required), the datalatches are implemented as a shift register so that the parallel datastored therein is converted to serial data for data bus 1620, and viceversa. In the preferred embodiment, all the data latches correspondingto the read/write block of m storage elements can be linked together toform a block shift register so that a block of data can be input oroutput by serial transfer. In particular, the bank of r read/writemodules is adapted so that each of its set of data latches will shiftdata in to or out of the data bus in sequence as if they are part of ashift register for the entire read/write block.

Additional information about the structure and/or operations of variousembodiments of non-volatile storage devices can be found in: (1) U.S.Patent Application Pub. No. 2004/0057287, “Non-Volatile Memory AndMethod With Reduced Source Line Bias Errors,” published on Mar. 25,2004; (2) U.S. Patent Application Pub No. 2004/0109357, “Non-VolatileMemory And Method with Improved Sensing,” published on Jun. 10, 2004;(3) U.S. patent application Ser. No. 11/015,199 titled “Improved MemorySensing Circuit And Method For Low Voltage Operation,” filed on Dec. 16,2004; (4) U.S. patent application Ser. No. 11/099,133, titled“Compensating for Coupling During Read Operations of Non-VolatileMemory,” filed on Apr. 5, 2005; and (5) U.S. patent application Ser. No.11/321,953, titled “Reference Sense Amplifier For Non-Volatile Memory,filed on Dec. 28, 2005. All five of the immediately above-listed patentdocuments are incorporated herein by reference in their entirety.

FIG. 19 illustrates an example of an organization of a memory array intoblocks for an all bit line memory architecture or for an odd-even memoryarchitecture. Exemplary structures of storage element array 1500 aredescribed. As one example, a NAND flash EEPROM is described that ispartitioned into 1,024 blocks. The data stored in each block can besimultaneously erased. In one embodiment, the block is the minimum unitof storage elements that are simultaneously erased. In each block, inthis example, there are 8,512 columns corresponding to bit lines BL0,BL1, . . . BL8511. In one embodiment referred to as an all bit line(ABL) architecture (architecture 1910), all the bit lines of a block canbe simultaneously selected during read and program operations. Storageelements along a common word line and connected to any bit line can beprogrammed at the same time.

In the example provided, four storage elements are connected in seriesto form a NAND string. Although four storage elements are shown to beincluded in each NAND string, more or less than four can be used (e.g.,16, 32, 64 or another number). One terminal of the NAND string isconnected to a corresponding bit line via a drain select gate, andanother terminal is connected to c-source via a source-side select gate.

In another embodiment, referred to as an odd-even architecture(architecture 1900), the bit lines are divided into even bit lines (BLe)and odd bit lines (BLo). In the odd/even bit line architecture, storageelements along a common word line and connected to the odd bit lines areprogrammed at one time, while storage elements along a common word lineand connected to even bit lines are programmed at another time. Data canbe programmed into different blocks and read from different blocksconcurrently. In each block, in this example, there are 8,512 columnsthat are divided into even columns and odd columns. In this example,four storage elements are shown connected in series to form a NANDstring. Although four storage elements are shown to be included in eachNAND string, more or fewer than four storage elements can be used.

During one configuration of read and programming operations, 4,256storage elements are simultaneously selected. The storage elementsselected have the same word line and the same kind of bit line (e.g.,even or odd). Therefore, 532 bytes of data, which form a logical page,can be read or programmed simultaneously, and one block of the memorycan store at least eight logical pages (four word lines, each with oddand even pages). For multi-state storage elements, when each storageelement stores two bits of data, where each of these two bits are storedin a different page, one block stores sixteen logical pages. Other sizedblocks and pages can also be used.

For either the ABL or the odd-even architecture, storage elements can beerased by raising the p-well to an erase voltage (e.g., 20 V) andgrounding the word lines of a selected block. The source and bit linesare floating. Erasing can be performed on the entire memory array,separate blocks, or another unit of the storage elements which is aportion of the memory device. Electrons are transferred from thefloating gates of the storage elements to the p-well region so that theV_(TH) of the storage elements becomes negative.

In the read and verify operations, the select gates (SGD and SGS) areconnected to a voltage in a range of 2.5 to 4.5 V and the unselectedword lines (e.g., WL0, WL1 and WL3, when WL2 is the selected word line)are raised to a read pass voltage, V_(PASS), (typically a voltage in therange of 4.5 to 6 V) to make the transistors operate as pass gates. Theselected word line WL2 is connected to a voltage, a level of which isspecified for each read and verify operation in order to determinewhether a V_(TH) of the concerned storage element is above or below suchlevel. For example, in a read operation for a two-level storage element,the selected word line WL2 may be grounded, so that it is detectedwhether the V_(TH) is higher than 0 V. In a verify operation for a twolevel storage element, with no boost structure, the selected word lineWL2 is connected to 0.8 V, for example, so that it is verified whetheror not the V_(TH) has reached at least 0.8 V. The source and p-well areat 0 V. The selected bit lines, assumed to be the even bit lines (BLe),are pre-charged to a level of, for example, 0.7 V. If the V_(TH) ishigher than the read or verify level on the word line, the potentiallevel of the bit line (BLe) associated with the storage element ofinterest maintains the high level because of the non-conductive storageelement. On the other hand, if the V_(TH) is lower than the read orverify level, the potential level of the concerned bit line (BLe)decreases to a low level, for example, less than 0.5 V, because theconductive storage element discharges the bit line. The state of thestorage element can thereby be detected by a voltage comparator senseamplifier that is connected to the bit line.

The erase, read and verify operations described above are performedaccording to techniques known in the art. Thus, many of the detailsexplained can be varied by one skilled in the art. Other erase, read andverify techniques known in the art can also be used.

FIG. 20 illustrates example threshold voltage distributions for thestorage element array when each storage element stores two bits of data.A first threshold voltage distribution E is provided for erased storageelements. Three threshold voltage distributions, A, B and C forprogrammed storage elements, are also depicted. In one embodiment, thethreshold voltages in the E distribution are negative and the thresholdvoltages in the A, B and C distributions are positive.

Each distinct threshold voltage range corresponds to predeterminedvalues for the set of data bits. The specific relationship between thedata programmed into the storage element and the threshold voltagelevels of the storage element depends upon the data encoding schemeadopted for the storage elements. For example, U.S. Pat. No. 6,222,762and U.S. Patent Application Pub. No. 2004/0255090, published Dec. 16,2004, both of which are incorporated herein by reference in theirentirety, describe various data encoding schemes for multi-state flashstorage elements. In one embodiment, data values are assigned to thethreshold voltage ranges using a Gray code assignment so that if thethreshold voltage of a floating gate erroneously shifts to itsneighboring physical state, only one bit will be affected. One exampleassigns “11” to threshold voltage range E (state E), “10” to thresholdvoltage range A (state A), “00” to threshold voltage range B (state B)and “01” to threshold voltage range C (state C). However, in otherembodiments, Gray code is not used. Although four states are shown, thepresent invention can also be used with other multi-state structuresincluding those that include more or less than four states.

Three read reference voltages, Vra, Vrb and Vrc, are also provided forreading data from storage elements. By testing whether the thresholdvoltage of a given storage element is above or below Vra, Vrb and Vrc,the system can determine what state the storage element is in.

Further, three verify reference voltages, Vva, Vvb and Vvc, areprovided. When programming storage elements to state A, the system willtest whether those storage elements have a threshold voltage greaterthan or equal to Vva. When programming storage elements to state B, thesystem will test whether the storage elements have threshold voltagesgreater than or equal to Vvb. When programming storage elements to stateC, the system will determine whether storage elements have theirthreshold voltage greater than or equal to Vvc.

In one embodiment, known as full sequence programming, storage elementscan be programmed from the erase state E directly to any of theprogrammed states A, B or C. For example, a population of storageelements to be programmed may first be erased so that all storageelements in the population are in erased state E. A series ofprogramming pulses such as depicted by the control gate voltage sequenceof FIG. 26 will then be used to program storage elements directly intostates A, B or C. While some storage elements are being programmed fromstate E to state A, other storage elements are being programmed fromstate E to state B and/or from state E to state C. When programming fromstate E to state C on WLn, the amount of parasitic coupling to theadjacent floating gate under WLn-1 is a maximized since the change inamount of charge on the floating gate under WLn is largest as comparedto the change in voltage when programming from state E to state A orstate E to state B. When programming from state E to state B the amountof coupling to the adjacent floating gate is reduced but stillsignificant. When programming from state E to state A the amount ofcoupling is reduced even further. Consequently the amount of correctionrequired to subsequently read each state of WLn-1 will vary depending onthe state of the adjacent storage element on WLn.

FIG. 21 illustrates an example of a two-pass technique of programming amulti-state storage element that stores data for two different pages: alower page and an upper page. Four states are depicted: state E (11),state A (10), state B (00) and state C (01). For state E, both pagesstore a “1.” For state A, the lower page stores a “0” and the upper pagestores a “1.” For state B, both pages store “0.” For state C, the lowerpage stores “1” and the upper page stores “0.” Note that althoughspecific bit patterns have been assigned to each of the states,different bit patterns may also be assigned.

In a first programming pass, the storage element's threshold voltagelevel is set according to the bit to be programmed into the lowerlogical page. If that bit is a logic “1,” the threshold voltage is notchanged since it is in the appropriate state as a result of having beenearlier erased. However, if the bit to be programmed is a logic “0,” thethreshold level of the storage element is increased to be state A, asshown by arrow 2100. That concludes the first programming pass.

In a second programming pass, the storage element's threshold voltagelevel is set according to the bit being programmed into the upperlogical page. If the upper logical page bit is to store a logic “1,”then no programming occurs since the storage element is in one of thestates E or A, depending upon the programming of the lower page bit,both of which carry an upper page bit of “1.” If the upper page bit isto be a logic “0,” then the threshold voltage is shifted. If the firstpass resulted in the storage element remaining in the erased state E,then in the second phase the storage element is programmed so that thethreshold voltage is increased to be within state C, as depicted byarrow 2120. If the storage element had been programmed into state A as aresult of the first programming pass, then the storage element isfurther programmed in the second pass so that the threshold voltage isincreased to be within state B, as depicted by arrow 2110. The result ofthe second pass is to program the storage element into the statedesignated to store a logic “0” for the upper page without changing thedata for the lower page. In both FIG. 20 and FIG. 21 the amount ofcoupling to the floating gate on the adjacent word line depends on thefinal state.

In one embodiment, a system can be set up to perform full sequencewriting if enough data is written to fill up an entire page. If notenough data is written for a full page, then the programming process canprogram the lower page programming with the data received. Whensubsequent data is received, the system will then program the upperpage. In yet another embodiment, the system can start writing in themode that programs the lower page and convert to full sequenceprogramming mode if enough data is subsequently received to fill up anentire (or most of a) word line's storage elements. More details of suchan embodiment are disclosed in U.S. Patent Application Pub. No.2006/0126390, titled “Pipelined Programming of Non-Volatile MemoriesUsing Early Data,” published Jun. 15, 2006, and incorporated herein byreference in its entirety.

FIGS. 22 a-c disclose another process for programming non-volatilememory that reduces the effect of floating gate to floating gatecoupling by, for any particular storage element, writing to thatparticular storage element with respect to a particular page subsequentto writing to adjacent storage elements for previous pages. In oneexample implementation, the non-volatile storage elements store two bitsof data per storage element, using four data states. For example, assumethat state E is the erased state and states A, B and C are theprogrammed states. State E stores data 11. State A stores data 01. StateB stores data 10. State C stores data 00. This is an example of non-Graycoding because both bits change between adjacent states A and B. Otherencodings of data to physical data states can also be used. Each storageelement stores two pages of data. For reference purposes, these pages ofdata will be called upper page and lower page; however, they can begiven other labels. With reference to state A, the upper page stores bit0 and the lower page stores bit 1. With reference to state B, the upperpage stores bit 1 and the lower page stores bit 0. With reference tostate C, both pages store bit data 0.

The programming process is a two-step process. In the first step, thelower page is programmed. If the lower page is to remain data 1, thenthe storage element state remains at state E. If the data is to beprogrammed to 0, then the threshold of voltage of the storage element israised such that the storage element is programmed to state B′. FIG. 22a therefore shows the programming of storage elements from state E tostate B′. State B′ is an interim state B; therefore, the verify point isdepicted as Vvb′, which is lower than Vvb.

In one embodiment, after a storage element is programmed from state E tostate B′, its neighbor storage element (WLn+1) in the NAND string willthen be programmed with respect to its lower page. For example, lookingback at FIG. 5, after the lower page for storage element 546 isprogrammed, the lower page for storage element 545 would be programmed.After programming storage element 545, the floating gate to floatinggate coupling effect will raise the apparent threshold voltage ofstorage element 546 if storage element 545 had a threshold voltageraised from state E to state B′. This will have the effect of wideningthe threshold voltage distribution for state B′ to that depicted asthreshold voltage distribution 2250 of FIG. 22 b. This apparent wideningof the threshold voltage distribution will be remedied when programmingthe upper page.

FIG. 22 c depicts the process of programming the upper page. If thestorage element is in erased state E and the upper page is to remain at1, then the storage element will remain in state E. If the storageelement is in state E and its upper page data is to be programmed to 0,then the threshold voltage of the storage element will be raised so thatthe storage element is in state A. If the storage element was inintermediate threshold voltage distribution 2250 and the upper page datais to remain at 1, then the storage element will be programmed to finalstate B. If the storage element is in intermediate threshold voltagedistribution 2250 and the upper page data is to become data 0, then thethreshold voltage of the storage element will be raised so that thestorage element is in state C. The process depicted by FIGS. 22 a-creduces the effect of floating gate to floating gate coupling becauseonly the upper page programming of neighbor storage elements will havean effect on the apparent threshold voltage of a given storage element.An example of an alternate state coding is to move from distribution2250 to state C when the upper page data is a 1, and to move to state Bwhen the upper page data is a 0.

Although FIGS. 22 a-c provide an example with respect to four datastates and two pages of data, the concepts taught can be applied toother implementations with more or less than four states and differentthan two pages.

FIG. 23 depicts a timing diagram describing a process for programmingnon-volatile memory. One possible programming process includes fourphases. For simplicity, in these and other figures, the duration of thephases are shown as being equal but, in practice, the duration of eachphase can be optimized based on experimental results. Further, specificvoltages to use can be optimized based on experimental results. Therelative voltage levels indicated are not necessarily to scale andprovide a high level guideline.

In one embodiment, as discussed, each NAND string can be provided withits own individually driven boost structure. The boost structures can beused during program, erase and/or read operations to provide couple avoltage to the storage elements. For example, the boost structures cancouple a significant amount of voltage to the floating gates of thestorage element to increase the total floating gate capacitance sincethe boost structures provide conductors to two additional facets of thestorage elements (e.g., the sides). A higher floating gate capacitanceresults in better retention and more immunity to disturb phenomena, asthe charge capacity increases and the Coulomb blockade effect isreduced. Furthermore, since a voltage on the boost structure is coupledto the storage elements, the voltages which are typically applied to thestorage elements via word lines can be reduced by the amount of thecoupled voltage. The voltages which are typically applied include theprogram voltage, V_(PGM), and pass voltages, V_(PASS). For example, thevoltage coupled to the floating gate of a selected storage element canbe expressed by the following equation:

V _(PGM) ×CR1+V _(BOOST STRUCTURE) ×CR2=V _(EFFECTIVE),   (1)

where CR1 is a coupling ratio of the program voltage, V_(PGM), to thefloating gate of a storage element, CR2 is the coupling ratio of theboost structure voltage, V_(BOOST STRUCTURE), to the floating gate, andV_(EFFECTIVE) is the effective or net voltage at the floating gate.Thus, with V_(EFFECTIVE) constant, V_(PGM) can be reduced byV_(BOOST STRUCTURE)×CR2/CR1. In one approach, V_(BOOST STRUCTURE) can beobtained from eqn. (1). Thus, the boost structure can assist inprogramming the storage elements. Further, the amount of assistanceprovided can be set independently for each NAND string by applying afixed boost voltage to one or more boost structures, then independentlydischarging the boost structures via the respective bit lines, asdiscussed in detail further below. The amount of discharge can becontrolled by the bit line voltage.

An additional advantage of the boost structure is that a portion of thecoupling to the floating gate of a storage element does not involve theinter-poly dielectric (e.g., see inter-poly dielectric layer 637, FIG.7). This is true since portions of the boost structure which providecoupling can extend close to the substrate and adjacent to the floatinggates (see, e.g., boost structure 1020 in FIG. 10), where the inter-polydielectric is not between the boost structure and the floating gate.Thus, the burden on the inter-poly dielectric is reduced. Similarly, thethickness of the inter-poly dielectric can be reduced. The coupling ofthe boost structure can be significant since the coupling occurs fromthree facets of the floating gate (e.g., from the top and both sides).

Further, the increased capacitance will increase the number of electronson the floating gate, thereby reducing the exacerbated retention anddisturb issues which may result from charge quantization effects infloating gates with very small capacitances.

It is also possible to concurrently verify the programming state ofmultiple storage elements associated with a word line, for instance,since a potential of each boost structure can be set independently. Inthis case, the boost structures can augment the voltage applied to thecontrol gates of the selected storage element via the word line bydifferent amounts for the different verify levels of a multi-levelstorage element. For example, conventionally the verify process involvesapplying a verify voltage Vva, Vvb or Vvc to the control gate of astorage element via a word line and determining whether the storageelement turns on. If it turns on, the storage element's V_(TH) is lessthan the verify voltage. If it does not turn on, the storage element'sonly one verify voltage can be applied at a time on a word line. Incontrast, with the boost structure provided herein, a fixed voltage,V_(CG-VERIFY), can be provided on the word line and V_(BOOST STRUCTURE)can be varied to provide the different verify voltages concurrently fordifferent storage elements on a common word line. For example, thecoupling from the boost structure to a control gate can be expressed bythe following equation:

V _(CG-VERIFY) +V _(BOOST STRUCTURE) ×CR3=V _(CG-EFFECTIVE),   (2)

where CR3 is a coupling ratio of the boost structure voltageV_(BOOST STRUCTURE) to the storage element's control gate, andV_(CG-EFFECTIVE) is the effective or net voltage received at the controlgate. The coupling ratio of V_(CG-VERIFY) is one when the control gateis provided by a portion of the word line. V_(BOOST STRUCTURE) cantherefore be set for the NAND strings associated with different storageelements to achieve V_(CG-EFFECTIVE)=Vva, Vvb or Vvc. See also FIG. 25.In one approach, V_(BOOST STRUCTURE) can be obtained from eqn. (2).

Generally, with bit line-by-bit line control of programming operations,and concurrent verification of multiple states, the number ofprogram-verify operations can be reduced from a number that covers thewidth of the natural distribution of programming characteristics plusthe programming window to a number that only covers the width of thenatural distribution of programming characteristics. Further, the numberof verify operations after each programming pulse can be reducedpotentially down to one. This increase in programming operation speedfacilitates the implementation of multi-level storage elements with anincreased number of states, e.g., eight states or more per storageelement. Also, as mentioned, a significant reduction in coupling effectsdue to the shielding function of the boost structure is also an enablingcomponent of such a multi-level storage element.

Moreover, the read process can be augmented by the boost structures in amulti-cycle read process in which the programming state of one or morestorage elements is determined in each cycle. The read process isanalogous to the verify process except multiple cycles may be needed asthe read state is not known in advance. By analogy to the verify processdiscussed above, the coupling from the boost structure to a control gatecan be expressed by the following equation:

V _(CG-READ) +V _(BOOST STRUCTURE) ×CR3=V _(CG-EFFECTIVE),   (3)

where V_(CG-READ) is the voltage on the word line in a given read cycle.V_(BOOST STRUCTURE) can therefore be set accordingly for the differentvalues of V_(CG-READ). See also FIG. 25. In one approach,V_(BOOST STRUCTURE) can be obtained from eqn. (3).

Further, the advantages provided by the boost structure in theprogramming, verifying and reading process can be achievedindependently. That is, the boost structure can be fully discharged sothat it has no effect during one or more of the programming, verifyingand reading processes.

Specifically, FIG. 23 depicts four phases: a first boosting phase, asecond boosting phase, a boost structure discharging phase and aprogramming phase. The cycle of four phases is repeated for eachprogramming pulse. Waveform 2300 depicts a voltage, V_(SOURCE), appliedto the source side of a NAND string. This voltage is a boost voltagesince it acts to boost a potential of the boost structure. As anexample, V_(SOURCE) can be about 8-10 V. Waveforms 2310 depict voltageswhich are applied to the select gates, including V_(SGS1), V_(SGS2) andV_(SGD). Waveform 2320 depicts a voltage on the unselected word lines,V_(UWL). Waveform 2330 depicts a voltage on the selected word line,V_(SWL). Waveforms 2340 depict a voltage on the bit line of the selectedNAND strings, V_(BL), in a first approach. Waveforms 2345 depict avoltage on the bit line of the selected NAND strings in a secondapproach. Waveforms 2350 depict a resulting voltage of the booststructure, V_(BOOST STRUCTURE), with the first or second approaches.

In the first boosting phase, the potential of the boost structure isincreased. In one approach, the source supply line is common to a groupof NAND strings, e.g., in a block, so the boost structure in each NANDstring is boosted at the same time and to the same extent. To enable thesource voltage, V_(SOURCE), to reach the boost structure, the firstselect gate, e.g., the outer select gate, which is closest to the sourcesupply line, is turned on, and the second select gate, e.g., the innerselect gate, is kept off. Thus, the NAND string is configured to receivethe boost voltage. In particular, V_(SGS1) is set at a level which issufficient to open the outer select gate. V_(SGS1) can exceed V_(SOURCE)by the threshold voltage of the outer select gate to turn the transistoron. The inner select gate can be maintained off by setting a lower valuefor V_(SGS2), e.g., lower than V_(SOURCE). V_(UWL), V_(SWL) and V_(BL)can all be set to 0 V. Waveforms 2350 depict how V_(BOOST STRUCTURE) isincreased when V_(SOURCE) is applied. The source supply line iselectrically coupled to the boost structure via the outer select gateand its associated source/drain regions.

A second boosting phase can optionally be used to further boost thepotential of the boost structure. In this phase, relatively high orelevated voltages, V_(HIGH), applied to the word lines are coupled tothe boost structure to further increase its potential. In one possibleapproach, V_(HIGH)=V_(PASS), the pass voltage used during programming.This will boost the boost structure to an even higher voltage as it isfloating in this phase. In particular, the outer select gate is turnedoff by dropping V_(SGS1), ensuring that the boost structure does notdischarge through the outer select gate, and V_(UWL) and V_(SWL) areincreased to V_(HIGH). Thus, both the outer and inner select gates areturned off. The increase in V_(BOOST STRUCTURE) is depicted by waveforms2350.

A third phase involves discharging of the boost structure. This providesthe ability to independently control V_(BOOST STRUCTURE) for each NANDstring. In this phase, the outer select gate is turned off while theinner select gate and the drain-side select gate are turned on. Forexample, the inner select gate and the drain-side select gate can beturned on by setting V_(SGS2) and V_(SGD), respectively, to a relativelyhigh level (waveform 2310). Further, V_(UWL) and V_(SWL) can bemaintained at the previous level to maintain all storage elements openduring the discharging. For example, this can be the highest read levelwhich is used. The level to which the boost structure discharges can becontrolled based on the applied bit line voltage, V_(BL), In a firstapproach, as indicated by waveforms 2340, V_(BL) can be set based on theprogramming state to which the selected storage element in the NANDstring is to be programmed. For example, state C can be the highestprogramming state (having the highest V_(TH)), state B can be anintermediate programming state and state A can be the lowest programmingstate. For state C, V_(BL) is set to a higher level to providerelatively little discharging of the boost structure, maintainingV_(BOOST STRUCTURE) at a relatively high level. For state B, V_(BL) isset to an intermediate level to provide an intermediate level ofdischarging, maintaining V_(BOOST STRUCTURE) at an intermediate level.For state A, V_(BL) is set to a low level to provide a high amount ofdischarging, maintaining V_(BOOST STRUCTURE) at a low level. Thespecific values of V_(BL) can be set based on the particularimplementation, including the level of V_(PGM) which is used. Thespacing between the specific values of V_(BL) may be non-linear.

In a second approach, as indicated by waveforms 2345, V_(BL) can be setto a fixed amplitude pulse, where the duration of the pulse varies basedon the programming state to which the selected storage element in theNAND string is to be programmed. Specifically, a longer pulse can beused for the higher programmed states, e.g., state C, while a shorterpulse is used for the lower programmed states, e.g., state A. Thus, thelevel to which the boost structure discharges can be set based on alevel and/or duration of a voltage applied to a drain side of the NANDstring.

Note also that V_(BOOST STRUCTURE) can vary as V_(PGM) varies withsuccessive programming pulses. A higher V_(BOOST STRUCTURE) cancompensate for a lower V_(PGM) and a lower V_(BOOST STRUCTURE) cancompensate for a higher V_(PGM). The decrease in V_(BOOST STRUCTURE) ineach case is depicted by waveforms 2350. In the example provided, theboost structure is discharged for all programmed states to a level inwhich some charge remains. In one option, for the highest programmedstate, e.g., state C, the boost structure need not be discharged at all.In another option, for the lowest programmed state, e.g., state A, theboost structure can be fully discharged.

The fourth phase is a programming phase in which V_(PGM) is applied tothe selected word line (waveform 2330). V_(PGM) can increase in astaircase manner with each successive programming pulse (FIG. 26). Withthe boost structures charged to various data-dependent voltages, the bitlines can now deliver the program or the inhibit voltages to the NANDstrings. Here, the inner and outer select gates are both turned off. Apass voltage V_(PASS) can be applied to the unselected word lines(waveform 2320), depending on the particular technique used. In someprogramming techniques, the source-side and/or drain-side neighboringunselected word lines receive different voltages, such as 0 V or otherlow voltage, than the other unselected word lines, which receiveV_(PASS). As mentioned, the voltage on the boost structure is coupled tothe floating gates of the storage elements to drive electrons into thefloating gate and increase the threshold voltage. The boost structureaugments the voltage coupled by the selected word line so the effect isthe same as if a larger V_(PGM) were used on the selected word line andas if a larger V_(PASS) were used on the unselected bit lines. However,the disadvantages of using a larger V_(PGM) or V_(PASS), such as programdisturb, are avoided. Alternatively, or additionally, the booststructure can reduce programming time. Note that optimal values for thevoltages shown can be determined by experimentation and will vary fordifferent memory devices.

After the program pulse is finished in the fourth phase, the word linesare brought down and a verification process is performed. The booststructure is floating and is coupled down by this action.

FIG. 24 depicts a timing diagram describing a process for verifying aprogramming state of non-volatile memory. Waveform 2400 depictsV_(SOURCE), waveforms 2410 depict V_(SGS1), V_(SGS2) and V_(SGD),waveform 2420 depicts V_(UWL), waveform 2430 depicts V_(SWL), waveforms2440 and 2445 depict V_(BL) in two different approaches, and waveform2450 depicts V_(BOOST STRUCTURE). In one embodiment, phases 1 and 2 arethe same as for the programming process of FIG. 23. Phase 3 can also beanalogous to the programming process of FIG. 23, except that V_(BL) canvary somewhat so that the boost structure is discharged according to theverify level of the selected storage elements.

In the fourth phase, the source-side select gates are turned on byraising V_(SGS1) and V_(SGS2), as depicted by waveforms 2410 andV_(SOURCE) is raised to the highest positive voltage required forverifying the highest programming state for the group of storageelements to be concurrently verified. V_(SGD) is also set so that thedrain-side select gate is turned on. The waveforms 2410 are offset forclarity. The word line voltage, V_(CG-VERIFY), is applied to eachstorage element to be verified via an associated word line. The effectof the boost structure is as if a different V_(CG-VERIFY) was applied toeach selected storage element according to the particular verify levelof the storage element. As a result, the storage elements can beconcurrently verified using different verify levels, resulting insignificant time savings. Thus, a single pass through the four phasescan be sufficient to characterize the programming state of all storageelements on a word line relative to a verify level. The number of statesin the group of states which can be concurrently verified depends, e.g.,on the V_(TH) separation of neighbor states, on the booststructure-to-floating gate capacitive coupling, and on the maximum rangeof allowed voltages on the assist gates. In some cases, an additionalverify pass may be used. If the threshold voltage of the storage elementselected for verifying is less than the verify level, the selectedstorage element will turn on. In this case, the storage element has notreached the target programming state, e.g., verify level, and requiresone or more additional programming pulses. If the threshold voltage ofthe storage element selected for verifying is greater than the verifylevel, the selected storage element will not turn on, indicating thatthe storage element has reached the target programming state and can belocked out from further programming.

Further, V_(BOOST STRUCTURE) discharges as indicated by waveforms 2450,and V_(BL) rises as indicated by waveforms 2445. The level to whichV_(BL) rises is indicative of a programming condition of the storageelement being verified. In particular, a verify process can be used asdiscussed in connection with FIG. 25.

FIG. 25 is a graph depicting a bit line voltage versus time relationshipfor different programming states. As mentioned, the boost voltage of theboost structure can be set based on a target verify level or a readlevel. During the verify phase, the bit lines are initially discharged,at least partly, and begin charging up during an integration time todifferent levels until a body bias causes the storage elements to stopcharging. The charging up of V_(BL) to a plateau is depicted in FIG. 25.The bit lines to be verified against different target states can besensed concurrently, reducing the number of verify operations down toone or two. In particular, curves 2500, 2510, 2520 and 2530 denoteV_(BL) when the storage element is in state C, state B, state A and theerased state, respectively. When curve 2500 exceeds a trip point 2505,the associated storage element shuts off. Similarly, when curve 2510exceeds a trip point 2515, the associated storage element shuts off,when curve 2520 exceeds a trip point 2525, the associated storageelement shuts off and when curve 2530 exceeds a trip point 2535, theassociated storage element shuts off. A determination that a storageelement has reached a target verify level can therefore be made duringthe verify phase. Similarly, a determination that a storage element hasreached a specific read level can be made during the read phase. Thus, aprogramming condition of a storage element can be determined bymonitoring V_(BL) during a verify or read process.

FIG. 26 depicts an example waveform applied to the control gates ofnon-volatile storage elements during programming. A voltage waveform2600 includes a series of program pulses 2610, 2620, 2630, 2640, 2650, .. . , that are applied to a word line selected for programming. In oneembodiment, the programming pulses have a voltage, V_(PGM), which startsat an initial level and increases by increments, e.g., 0.5 V, for eachsuccessive programming pulse until a maximum level is reached. Asmentioned, the level of V_(PGM) can advantageously be reduced with theuse of a boost structure as discussed herein. In between the programpulses are verify pulses 2615, 2625, 2635, 2645, 2655, . . . . Further,in some embodiments, only one verify pulse need be used. In otherembodiments, there can be additional verify pulses. The verify pulsescan have an amplitude of V_(CG-VERIFY) as discussed previously (see FIG.24).

FIG. 27 depicts a timing diagram describing a process for reading aprogramming state of non-volatile memory. Waveform 2700 depictsV_(SOURCE), waveforms 2710 depict V_(SGS1), V_(SGS2) and V_(SGD),waveform 2720 depicts V_(UWL), waveform 2730 depicts V_(SWL), waveforms2740 and 2745 depict V_(BL), in two different approaches, and waveform2750 depicts V_(BOOST STRUCTURE). The first two phases and the fourthphase are analogous to the verify process of FIG. 24. Phase 3 can alsobe analogous to the verify process of FIG. 24, except that the leveland/or duration V_(BL) can vary somewhat so that the boost structure isdischarged according to the read level, which can vary from the verifylevel (see, e.g., FIG. 20). In the fourth phase, the source-side selectgates are turned on by raising V_(SGS1) and V_(SGS2), as depicted bywaveforms 2710 and V_(SOURCE) is raised to the highest positive voltagerequired for reading the highest programming state for the group ofstorage elements to be concurrently read. V_(SGD) is also set so thatthe drain-side select gate is turned on. The waveforms 2710 are offsetfor clarity.

The read process is analogous to the verify process except that a cyclethrough the phases is performed for each read level since theprogramming state may be tested against multiple read levels rather thana single verify level. In particular, the first through third phases canbe the same as discussed above in connection with the programming andverify processes, except that all of the NAND strings are discharged tothe same level, in one embodiment, based on the read level (e.g., Vra,Vrb or Vrc) for state A, B or C, for instance (waveform 2740) of thecurrent read cycle. Further, the fourth phase can be the same asdiscussed in connection with the fourth phase of the verify process ofFIG. 24, except that the selected word line voltage is V_(CG-READ),which varies for each read cycle. For example, V_(CG-READ) can have avalue which is based on the read level. In one approach,V_(CG-READ)=Vra, Vrb or Vrc (FIG. 20) less the control gate couplingprovided by the boost structure.

FIG. 28 depicts a timing diagram for unselected bit lines duringprogramming, verifying or reading of non-volatile memory. As indicatedby waveform 2800, V_(SGS1) and V_(SGS2) can be maintained at 0 V to keepthe source-side select gates turned off. V_(SGD) (waveform 2800) andV_(BL) (waveform 2810) can be maintained at a relatively low, comparablelevels to keep the drain-side select gate turned off. As a result, theboost voltage V_(SOURCE) is not coupled to the boost structure, soV_(BOOST STRUCTURE) remains at 0 V.

FIG. 29 is a flow chart describing one embodiment of a process forprogramming non-volatile memory. In one implementation, storage elementsare erased (in blocks or other units) prior to programming. In step2900, a “data load” command is issued by the controller and inputreceived by control circuitry 1610 (FIG. 16). In step 2905, address datadesignating the page address is input to decoder 1614 from thecontroller or host. In step 2910, a page of program data for theaddressed page is input to a data buffer for programming. That data islatched in the appropriate set of latches. In step 2915, a “program”command is issued by the controller to state machine 1612.

Triggered by the “program” command, the data latched in step 2910 willbe programmed into the selected storage elements controlled by statemachine 1612 using the stepped pulses 2610, 2620, 2630, 2640, 2650, . .. of FIG. 26 applied to the appropriate word line. In step 2920, theprogram voltage, V_(PGM), is initialized to the starting pulse and aprogram counter PC maintained by state machine 1612 is initialized atzero. In step 2925, a programming process begins (see also FIG. 23).Specifically, at step 2930, a common boosting of the boost structures isperformed. At step 2935, additional boosting of the boost structures canbe performed. At step 2940, individual discharging of boost structuresoccurs based on the target programming state of the associated selectedstorage elements. At step 2945, programming of the selected storageelements occurs by applying V_(PGM) on the selected word line. If logic“0” is stored in a particular data latch indicating that thecorresponding storage element should be programmed, then thecorresponding bit line is grounded. On the other hand, if logic “1” isstored in the particular latch indicating that the corresponding storageelement should remain in its current data state, then the correspondingbit line is connected to V_(DD) to inhibit programming.

At step 2950, a verify process begins. Specifically, at step 2955, acommon boosting of the boost structures is performed. At step 2960,additional boosting of the boost structures can be performed. At step2965, individual discharging of boost structures occurs based on thetarget verify level which can differ among the storage elements. At step2970, verifying of the selected storage elements occurs by applyingV_(CG-VERIFY) on the selected word line and characterizing a programmingstate of the selected storage elements such as by determining whetherthe selected storage elements turn on (see also FIGS. 24 and 25). If itis detected that the target threshold voltage of a selected storageelement has reached the appropriate verify level, then the data storedin the corresponding data latch is changed to a logic “1.” If it isdetected that the threshold voltage has not reached the appropriateverify level, the data stored in the corresponding data latch is notchanged. In this manner, a bit line having a logic “1” stored in itscorresponding data latch does not need to be programmed further. Whenall of the data latches are storing logic “1,” the state machine (viathe wired-OR type mechanism described above) knows that all selectedstorage elements have been programmed.

In step 2975, a determination is made as to whether all of the datalatches are storing logic “1.” If so, the programming process iscomplete and successful because all selected storage elements wereprogrammed and verified, and a status of “PASS” is reported in step2980. If, in step 2975, it is determined that not all of the datalatches are storing logic “1,” then the programming process continues.In step 2985, the program counter PC is checked against a program limitvalue PCmax. One example of a program limit value is twenty; however,other numbers can also be used. If the program counter PC is not lessthan PCmax, then the program process has failed and a status of “FAIL”is reported in step 2990. If the program counter PC is less than PCmax,then V_(PGM) is increased by the step size and the program counter PC isincremented in step 2995. After step 2995, the process loops back tostep 2925 to apply the next programming pulse.

FIG. 30 is a flow chart describing one embodiment of a process forreading a non-volatile memory. The read process begins at step 3000 andcan include a number of read cycles, one for each read state. In a firstread cycle, step 3010 includes selecting a read level, e.g., Vrc, Vrb orVra (see FIG. 20). Additional read cycles can be performed when thestorage elements include additional states, e.g., eight states insteadof four. In one approach, the process begins with the highest readstate, e.g., state C which has a read verify level of Vrc. Step 3020includes performing common boosting of the boost structures while step3030 includes performing additional boosting of the boost structures.Step 3040 includes discharging the boost structures to a common levelbased on the current read level. Step 3050 includes setting a controlgate voltage V_(CG-READ) based on the current read level, and step 3060includes applying V_(CG-READ) to the selected word line. Step 3070includes characterizing the programming states of the storage elementsrelative to the current read level, e.g., by determining whether thestorage elements turn on when V_(CG-READ) is applied. For instance, if astorage element does not turn on, it can be concluded that it is instate C. At decision block 3080, a determination is made as to whetherthere are additional read cycles remaining. If an additional cycle isremaining, the process continues at step 3010 by selecting the next readlevel. The process continues until there are no additional read cycles,at which point the read process ends (step 3090).

The foregoing detailed description of the invention has been presentedfor purposes of illustration and description. It is not intended to beexhaustive or to limit the invention to the precise form disclosed. Manymodifications and variations are possible in light of the aboveteaching. The described embodiments were chosen in order to best explainthe principles of the invention and its practical application, tothereby enable others skilled in the art to best utilize the inventionin various embodiments and with various modifications as are suited tothe particular use contemplated. It is intended that the scope of theinvention be defined by the claims appended hereto.

1. A method for fabricating non-volatile storage, comprising: forming a set of storage elements, at least in part, on a substrate; forming inner and outer select gates at a first end of the set of storage elements; and forming a boost structure extending along the set of storage elements and in communication with the substrate at a location between the first and second select gates.
 2. The method of claim 1, wherein: the location at which the boost structure communicates with the substrate comprises a source/drain region.
 3. The method of claim 1, wherein: the boost structure comprises an elongated conductive material.
 4. The method of claim 1, wherein: the set of storage elements and the inner and outer select gates are provided in a NAND string.
 5. The method of claim 4, wherein: the inner and outer select gates are provided at a source side of the NAND string.
 6. The method of claim 1, wherein: the boost structure comprises a portion that extends toward the substrate adjacent to a floating gate of at least one of the storage elements for shielding the floating gate from an adjacent floating gate of an adjacent storage element.
 7. A method for fabricating non-volatile storage, comprising: forming a plurality of NAND strings on a substrate; forming inner and outer select gates at a first end of each NAND string; and forming a plurality of boost structures, each boost structure extending along a respective NAND string and in communication with the substrate at a respective location between the first and second select gates of the respective NAND string.
 8. The method of claim 7, wherein: each boost structure is independently controllable during programming operations via voltages applied to the respective NAND strings.
 9. The method of claim 7, wherein: the respective locations of the substrate comprise source/drain regions.
 10. The method of claim 7, wherein: the inner and outer select gates are provided at a source side of each NAND string.
 11. The method of claim 7, further comprising: forming control lines for concurrently applying a common boost voltage to each boost structure via the respective NAND strings.
 12. The method of claim 11, wherein: the control lines control the inner and outer select gates of each NAND string and supply the common boost voltage to each boost structure via the first end of each NAND string.
 13. The method of claim 7, further comprising: forming control lines for independently discharging each boost structure, at least in part.
 14. The method of claim 13, further comprising: forming a select gate at a second end of each NAND string, the control lines control the select gate at the second end of each NAND string to perform the independent discharging.
 15. The method of claim 13, wherein: the control lines include bit lines which supply voltages to a second end of each NAND string for controlling a level to which each boost structure is independently discharged.
 16. A method for fabricating non-volatile storage, comprising: forming a plurality of NAND strings on a substrate; forming a plurality of boost structures, each boost structure extending along a respective NAND string; and forming control lines for concurrently applying a common boost voltage to each boost structure, and for independently discharging each boost structure, at least in part, during programming operations.
 17. The method of claim 16, further comprising: forming inner and outer select gates at a first end of each NAND string, the boost structures communicate with the substrate at a respective location between the inner and outer select gates of each NAND string.
 18. The method of claim 17, wherein: the control lines control the inner and outer select gates of each NAND string and supply the common boost voltage to each boost structure via the first end of each NAND string.
 19. The method of claim 16, further comprising: forming inner and outer select gates at a first end of each NAND string, forming a select gate at a second end of each NAND string, the control lines control the select gate at the second end of each NAND string to perform the independent discharging.
 20. The method of claim 16, wherein: the control lines include bit lines which supply voltages to each NAND string for controlling a level to which each boost structure is independently discharged. 